Ca Isotope Composition Of Cenozoic Basalts In Zhejiang And Fujian Areas:Implications For Source Compositions Of The Cenozoic Basalts In The South China

In this paper,the Ca isotopic composition of Cenozoic basalts in Zhejiang-Fujian region is analyzed,in combination with the major elements,trace elements and Sr-Nd-Pb isotopic compositions of the whole rock,the source properties and petrogenesis of Cenozoic basalts in Zhejiang-Fujian region are discussed.According to the correlation between Mg O and major elements(trace elements)and Sr-Nd isotope and major elements(trace elements),Zhejiang-Fujian basalts can be divided into four different genetic types: Longyou,Quzhou basalts in Zhejiang inner belt,Quzhou basalt in Fujian inner belt;Xinchang-Shengzhou basalts in Zhejiang middle belt;Pan'an basalts in Zhejiang middle belt,Linhai basalts in Zhejiang outer belt and Niutoushan basalt in Fujian outer belt.The Sr(Nd)isotopic compositions of the Xinchang-Shengzhou basalts are negatively(positively)correlated with whole rock Ce/Pb,Zr/Hf,Ca/Al ratios and are positively(negatively)correlated with Ti*(Ti*=2×Ti N/(Eu N+Gd N),where N represents primitive mantle normalized value).The observation indicates that the Xinchang-Shengzhou basalts are derived from heterogeneous mantle sources and their compositional variations can be explained by mixing of two parental magmas.We infer that the two mantle sources are carbonated lithospheric mantle and FOZO-like mantle component similar to the mantle source of the Hainan basalts,respectively.Longyou,Quzhou and Mingxi basalts in the inner zone also have the same correlation as Xinchang-Shengzhou basalts,but their composition varies in a small range,and they are located near Xinchang-Shengzhou alkaline basalts.This may indicate that the basalts in the inner zone are mainly controlled by magma from the carbonated lithospheric mantle,while the proportion of FOZO-like components in the genesis of basalts in the inner zone may be small.Pan'an and Linhai basalts have anomalously significant K,Pb,Zr,Hf and Ti negative anomalies.They also have the highest Ca O content and Ca O/Al2O3 ratio,the lowest Si O2 and Al2O3 content and the lowest Ti* value(< 0.6,Ti* value of Linhai basalts are < 0.4)and the highest Zr/Hf,Ca/Al(>1.5)ratio in all samples.Moreover,Linhai and Panan basalts are located in the carbonated lithospheric mantle in isotope-related diagrams.Therefore,Pan'an and Linhai may be directly formed by partial melting of the carbonated lithospheric mantle,the contribution of FOZO-like components may be negligible.Niutoushan basalt in the outer zone of Fujian Province may undergo fractional crystallization of clinopyroxene,garnet and Fe-Ti oxides.The basalts does not have negative anomalies of K,Zr and Hf,but only negative anomalies of Pb and Ti.They also haves the lowest Mg O and Ca O/Al2O3 contents and the highest Si O2 and Al2O3 contents in all samples.In the isotope diagram,Niutoushan basalt falls in the area of FOZO,which also indicates that Niutoushan basalts come from partial melting of FOZO-like components.The predecessors have done a lot of research on the factors affecting the fractionation of Ca isotopes in basalts.They believe that post-magmatic alteration and contamination of the crust will lead to the fractionation of Ca isotopes.Separation crystallization has little effect on the fractionation of Ca isotopes.There is still a great controversy about the effect of partial melting on the fractionation of Ca isotopes.Some scholars believe that partial melting in larger(e.g.,25%~30%)Melting conditions may lead to significant fractionation of Ca isotopes.Most scholars believe that carbonate added to the mantle will lead to significant fractionation of Ca isotopes.We firstly reported Ca isotopic compositions of the Cenozoic basalts in Zhejiang-Fujian region.The ?44/40 Ca values(0.59±0.03-0.72±0.02,with average value of 0.66±0.07‰)of these basalts are much lower than Bulk Silicate Earth(0.94±0.05‰),normal mid-ocean ridge basalts and ocean island basalts.After excluding the effects of post-magmatic alteration,crustal contamination and fractional crystallization on Ca isotope fractionation,we believe that the abnormally low ?44/40 Ca values of the Xinchang-Shengzhou and Longyou,Quzhou,Mingxi basalts may be inherited from two parental magmas with similar and lower ?44/40 Ca values.The lower value of ?44/40 Ca of Pan'an and Linhai basalts in Zhejiang Province inherits from the carbonated lithospheric mantle.The lower value of ?44/40 Ca of Niutoushan basalts in the outer zone of Fujian Province is due to partial melting of FOZO-like components.We simulated the effect of carbonation on the Ca isotope composition of lithospheric mantle.The simulation results show that when the value of ?44/40 Ca of carbonate is less than-0.3,the value of ?44/40 Ca of lithospheric mantle decreases by more than 0.2 when 1% carbonate is added,and when 2% carbonate is added,the value of ?44/40 Ca of lithospheric mantle decreases by more than 0.35.With the increase of the ?44/40 Ca value of the carbonate,more carbonate may be needed to reduce the ?44/40 Ca value of the lithospheric mantle to the ?44/40 Ca value of the Zhejiang-Fujian basalts.